The present invention relates to chirped-frequency oscillators and, more particularly, to novel methods and apparatus for digital automatic generation of the chirped-frequency signal and for automatic correction of average frequency and sweep rate thereof.
In co-pending application Ser. No. 331,491, filed Dec. 17, 1981, assigned to the assignee of the present invention and incorporated herein by reference in its entirety, methods and circuitry for chirped oscillator automatic frequency control are described and claimed. This circuitry utilizes analog subcircuits for generation of a swept-frequency signal waveform from a chirped oscillator. In one intended use, with power-line-carrier transceivers, the circuitry (located in each of a plurality of stations of a distributed system) must generate the appropriate chirped signal, without adjustment or maintenance, over a period of many years. The stability requirements for the chirped oscillator in each transceiver of such a system requires that long term drift in certain elements of the control loops be reduced or eliminated. The circuitry of the aforementioned application obtains correction signals by comparing fixed reference voltages with the voltage output from digital-to-analog converters following a set of counters. Long term drift in the reference voltages applied to the digital-to-analog converter and/or in the fixed reference voltages cause output frequency errors. Thus, while frequency drifts in an analog ramp generator and a voltage-controlled oscillator can be corrected, the circuitry of the co-pending application contains numerous analog portions in its error detecting circuitry, which analog portions have certain drifts which cannot be corrected for over the required period of time. It is therefore desirable to provide methods and circuitry for automatic frequency control of a chirped oscillator which utilizes digital means for deriving corrective error signals of binary nature, to provide the required long-term stability. It is further desirable to provide digital automatic frequency control circuitry and methods in which the only source of error is the error, if any, of a common time-base element.